During a well cementing operation, after a casing string is set in a borehole, cement slurry needs to be pumped into the borehole in order to pack the layers of oil, gas and water and also to sustain the casings. Under the pressure from a pump, the pumped cement slurry flows upward to the annulus between the walls of the casings and the borehole after it reaches the bottom of the borehole through the casings.
However, in the course of a cementing operation in the above mentioned borehole, when the column height of the cement slurry inside the casing reaches a certain value, the difference in density between the fluids inside and outside of the casings will cause the cement slurry to move without the help of the pump pressure. Under such conditions, the pumped flow rate is unequal to the flow rate returning upward to the ground surface, which often results in occurrence of mud penetration and seriously deteriorated cementing quality.
In a conventional cementing operation, a ground pump flow recorder is often used to sense the flow rate of the pumped cement slurry in order to control the flow rate thereof, but the disadvantage of the prior art is that the flow rate of the cement slurry is not determined quickly enough at the moment of the cementing operation to compensate for unequal flow rates.
Therefore, the object of the invention is to overcome the disadvantage of the prior art. The invention provides a downhole cut-off valve which automatically can adjust the flow rate of a cement slurry in response to fluctuations in the difference between flow rates of pumped and return cement slurry.